DR. DAUBENY ON THE ROTATION OF CROPS, ETC. 181 



As I proceeded however in my experiments, I began to find, that both the postu- 

 lates on which I had built were unsound, for neither was I able to detect any foreign 

 organic matter in the soil, referable to the excretions of the crop which had grown 

 in it*, nor did I find that uniform difference between the shifting and the permanent 

 crop, to the disadvantage of the latter, which I should have expected upon the prin- 

 ciples of De Candolle's theory. 



Moreover, the researches of Braconnot, which have since been made known to the 

 world, tended still further to throw doubt upon the truth of the facts on which the 

 doctrine of excretion reposed, and when no longer swayed by the authority of the 

 distinguished author of the theory in question, I perceived more clearly the difficulty 

 of reconciling it with many facts or opinions that seemed current amongst agricul- 

 turists — such, for instance, as the growth of repeated crops of the most exhausting 

 plants in certain rich alluvial, or newly settled countries ; the continuance of a plant 

 in a state of nature for ages in the same locality ; and lastly, the views of Liebig, 

 which went to prove, that the food of plants, so far as their organic constituents are 

 concerned, is derived in all instances from the elements of air and of water. 



No sooner, therefore, had I become suspicious as to the truth of the opinion which 

 I had previously entertained as to the excretions from the roots of plants being 

 capable of explaining the falling off of a crop after repetition, than I felt desirous of 

 shaping my inquiries in such a manner as to ascertain, if possible, which of the other 

 two conceivable explanations might deserve a preference ; whether, for instance, the 

 falling off of the crop was attributable to a failure in the soil of organic matters fitted 

 for its nutrition, or of those inorganic materials which it equally required. 



* The soils that seemed to me most likely to afford indications of the presence of root excretions were those 

 which had reared crops of poppies and of tobacco for several years in succession, the former plant containing, 

 in morphia and meconic acid, products readily recognizable by chemical tests, and the latter one sufficiently so 

 in nicotine. 



I accordingly digested sifted portions of the soils, amounting in each instance to 5 lbs., in water for several 

 hours. 



The water drained off was evaporated, and then filtered. 



The clear solution was first treated with sugar of lead, and the precipitate which fell was collected, and then 

 dissolved in water acidulated with sulphuric acid. Had any meconic acid existed in combination with the lead, 

 it would have been thus separated, the metal being precipitated along with the sulphuric acid with which it 

 forms an insoluble salt. 



None of the tests, however, usually employed for detecting meconic acid produced any effect, — chloride of 

 iron dissolved in alcohol causing no red colour, and ammonio-sulphate of copper not being rendered green. 



The liquor remaining after the introduction of the sugar of lead might have contained morphia held in 

 solution by acetic acid. To detect it, the lead was in the first place thrown down by sulphuretted hydrogen, 

 after removing which, the remaining solution, after being concentrated, was treated with ammonia, which pro- 

 duced a flocculent precipitate. 



This, however, proved destitute of morphia, for neither was there any blue colour as produced by chloride of 

 iron, nor any redness by nitric acid. 



My attempts to detect nicotine in the soil in which tobacco had been grown proved equally ineflFectual. 



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